Optical Power Limiter

The present invention relates broadly to an optical power limiter, in particular to a miniaturised optical power limiter for quantum and classical optical communication with improved performance.

Any mention and/or discussion of prior art throughout the specification should not be considered, in any way, as an admission that this prior art is well known or forms part of common general knowledge in the field.

(1) PCT/SG2021/050403: An optical power limiter design using free space optics to launch light into an active medium, in which the power limiting is based on the thermo-optical defocusing effect in the active medium. When the input light beam has a relatively high optical power, the absorption-induced reflective index gradient in the active medium works as a concave lens and defocuses the light beam. In this way, the device can dynamically control the amount of optical power transmitted through a diaphragm. As a result, the final output optical power is limited even with the increase of the input optical power. This design is bulky and weighty, and has a relatively large insertion loss. (2) Fiber-optical power limiter based on liquid core optical fiber (IEEE Photonics Technology Letters 24, 297-299, (2011)): The power limiting effect in a liquid-core optical fiber (LCOF) has been proposed, where the absorption of evanescent field in the thin absorption layer deposited on the LCOF cladding will cause heat accumulation, which increases the temperature of the fiber. Since the thermal-optical coefficients of the core and cladding experience differential heating, this decreases the transmission efficiency of the light propagating in the fiber core and limits the final output power. Liquid-core optical fibers are not commercially available and will incur a large manufacturing cost. (3) Fiber-optical power limiter based on optical adhesive (Applied Optics 40, 6611 (2001)): In this paper, the output power limiting is achieved by utilizing the thermal-optical effect of the optical adhesive connecting the two fiber collimators. Connection of two fiber collimators with optical adhesive is a technically demanding endeavor, which involves consistent UV curing of the adhesive at every point, precise calibration of the collimators caused by adhesive shrinkage, etc. (4) Optical power limiter based on photonic chip micro-ring resonator (Scientific Reports 4, 6676, (2014)): Chip-based micro-ring resonator has a specific working frequency. When the absorption of the input light increases the temperature of the ring resonator, its working wavelength will shift and therefore attenuates the input light. Therefore, the power limiting effect applies only on a specific working wavelength of the input signal and has limited power limitation on the output power. A number of optical power limiter designs have been reported.

Embodiments of the present invention seek to address at least one of the above problems.

a first optical mode altering element configured to receive an input optical signal from a first waveguide; and an active medium coupled to the first optical mode altering element at a first end of the active medium such that a mode altered optical signal based on the input optical signal can enter the active medium, wherein a second end of the active medium is configured to couple the mode altered optical signal into a second waveguide as an optical output signal; wherein the active medium has a thermo-optic coefficient such that the mode altered optical signal entering the active medium experiences a refractive index gradient in the active medium as a result of absorption; and wherein the power of the optical output signal coupled into the second waveguide is limited to a maximum power value based on mode overlap of the mode altered optical signal and the second waveguide. In accordance with a first aspect of the present invention, there is provided an optical power limiter comprising:

configuring a first optical mode altering element to receive an input optical signal from a first waveguide; coupling an active medium to the first optical mode altering element at a first end of the active medium such that a mode altered optical signal based on the input optical signal can enter the active medium; and configuring a second end of the active medium to couple the mode altered optical signal into a second waveguide as an optical output signal; wherein the active medium has a thermo-optic coefficient such that the mode altered optical signal entering the active medium experiences a refractive index gradient in the active medium as a result of absorption; and wherein a power of the optical output signal coupled into the second waveguide is limited to a maximum power value based on mode overlap of the mode altered optical signal and the second waveguide. In accordance with a second aspect of the present invention, there is provided a method of fabricating an optical power limiter comprising:

In accordance with a third aspect of the present invention, there is provided an optical device or system comprising the power limiter in of the first aspect.

In accordance with a fourth aspect of the present invention, there is provided a method of limiting optical power using the power limiter of the first aspect.

1. Miniature size: the optical power limiter according to an example embodiment is able to achieve optical power limiting with a beam size of micro-meter, which is suitable for waveguide implementation, for example optical fiber and photonic integrated chips. 2. Minimal insertion loss: the optical power limiter according to an example embodiment optimises the optical mode coupling in the low input power case. So the insertion loss in low input power scenarios are minimal. 3. Adjustable power limiting threshold (i.e. maximum output optical power): the optical power limiter according to an example embodiment provides configurable system parameters to adjust the power limiting threshold. 4. Minimal distortion on the signal: The optical power limiter according to an example embodiment adds only attenuation on the input optical signal, and introduces minimal impact (if not negligible impact) on the intensity, phase, or polarization degrees of freedom of the input optical signal. Embodiments of the present invention provide an optical power limiter. Embodiments of the present invention can have several advantages, including, but not limited to, one or more of:

1. Optical communication 2. Sensing 3. Quantum cryptography Industrial applications of embodiments of the present invention include, for example, to the following:

Optical power limiters according to example embodiments focus on waveguide input and output, and handle the input beam with a much smaller beam size compared to existing proposals such as PCT/SG2021/050403. This advantageously gives a greater power density in the active medium, leading to a stronger thermo-optical defocusing effect, which results in a shorter transmission distance and a smaller absorption loss. Moreover, different from the proposal in PCT/SG2021/050403 where free-space optical transmission is considered, embodiments of the present invention are based on the mode overlap condition as a replacement of diaphragm. By managing the mode overlap conditions for the input and output light, one can advantageously optimise the power limiting threshold and the insertion loss relations of the optical power limiter according to example embodiments for the best performance.

100 102 104 106 108 110 102 104 106 108 102 104 106 108 100 110 104 110 1 FIG. A first optical power limiteraccording to an example embodiment is shown in, which comprises the input and output waveguide,, taper,and the active medium. The waveguide,can be optical fiber, waveguide in photonic integrated circuit (PIC), etc in various example embodiments. The taper,is an optical structure which can alter the optical mode of the input and output light from the waveguides,. Example of the taper,can be the core expansion in the optical fiber, taper structure, and large core waveguide mode converter in PIC platforms, in various example embodiments. In the optical power limiter, one can alter the beam parameters in the active mediumand manage the optical mode overlap of the output waveguide. The active mediumis where the thermo-optical defocusing effect takes place.

200 202 203 202 202 204 206 2 FIG. A second optical power limiteraccording to an example embodiment is shown in. The taper is replaced with an optical structurefor coupling to the input waveguide, which can alter the optical mode and the beam parameters. The optical structurecan be micro lenses, Gradient-Index (GRIN) Lenses, collimators, and the waveguide version of all the previous mention structures, etc, in various example embodiments. The function of this optical structurecan be light focusing inside the active medium, which gives a modifiable beam waist and focal length. In this way, adjustable system parameters (power limiting threshold, insertion loss) can be achieved in the coupling to the output waveguide.

300 301 302 303 304 306 304 306 304 3 FIG. A third optical power limiteraccording to an example embodiment is shown in. A first optical structurefor coupling to the input waveguideis provided, which can alter the optical mode and the beam parameters. A second optical structureis placed at the output side for coupling to the output waveguide, to optimize the mode overlap between the optical mode after the active mediumand the output waveguide. In this way, the mode mismatch (and the insertion loss) is minimal for the low input power case. While in the high input power case, the light beam in the active mediumwill diverge because of the thermo-optical defocusing effect, leading to a significant mode mismatch in the light coupling to the output waveguide, which limits the output optical power.

306 Furthermore, the length of the active mediumcan be adjusted to obtain different power limiting thresholds. In this way, adjustable power limiting threshold with minimal insertion loss can be achieved.

2 3 FIGS.and Non-limiting example embodiments according toare using single mode fiber (SMF28, mode field diameter: 10.4 μm @ 1550 nm) as the input and output waveguide, GRIN lenses as the input and output optical structure, and optical adhesive with negative TOC as the active medium.

200 300 2 FIG. 3 FIG. Based on this configuration, experiments were conducted for the optical power limiter() and the optical power limiter(). The single mode fiber is a commonly used waveguide for optical communication and quantum cryptography, which could be easily integrated in the fiber optical systems. The GRIN lens has a compact size of only ~mm in diameter and length, and the optical adhesive is also widely used in optical systems. Both GRIN lenses and optical adhesive are cost-effective and easily accessible.

200 4 FIGS.A-C The experimental results of the optical power limiterare shown in. By configuring the beam width (3 μm to 7.3 μm) and the medium length (100 μm to 300 μm), adjustable power limiting threshold (10.53 dBm (11.3 mW)-17.3 dBm (53.7 mW)) and insertion loss (3.1 dB-12.6 dB) are achieved.

300 306 5 FIG. The experimental results of the optical power limiterare shown in. By changing the mediumlength (distance between two GRIN lenses, from 6.5 mm to 15.8 mm), the power limiting threshold is adjustable from 13.36 dBm (21.7 mW) to 21.25 dBm (133.4 mW), with an insertion loss changing from 1.89 dB to 2.9 dB for the low input power case. The insertion loss could be further reduced by improving the mode-matching and the interface reflections, in different example embodiments.

As described above, it was demonstrated that embodiments of the present invention can achieve a miniaturized power limiting effect with a much smaller footprint (~mm) compared to existing proposal such as PCT/SG2021/050403 (~10 cm), minimal insertion loss (~1.89 dB) compared to existing proposal such as PCT/SG2021/050403 (~5.1 dB), and an adjustable power limiting threshold.

The optical power limiters according to example embodiments can be useful in various industrial applications, for example in quantum cryptography and optical communication:

In quantum cryptography, the optical power limiter according to an example embodiment can be used as a countermeasure against trojan-horse attack by limiting the energy of the eavesdropping light, potential countermeasure against plug-and-play QKD with untrusted light sources, and potential counter measure against bright illumination attacks including laser damage attacks and detector blinding attacks [PRX QUANTUM 2, 030304 (2021)].

With features of low insertion loss and a small footprint, embodiments of the present invention can work as a general component for protecting quantum cryptography systems. For example, the small insertion loss and the compact size enable a higher level of system integration, especially for the receiver side.

In the most widely deployed BB84 Quantum Key Distribution (QKD) system, it has been shown that the receiver is one of the most vulnerable parts of the whole system, where the detectors could be manipulated by a strong eavesdropping light [Nat Photon 4, 686 (2010), Rev. Mod. Phys. 92, 025002 (2020)]. A standard countermeasure to such an attack is to actively monitor the input light power. However, it has been shown that the monitoring devices could also be hacked by laser-damage attack. In this case, the calibrated system parameters can be changed, and the monitoring device can be damaged. How to tackle this problem is an open problem to the field [Phys. Rev. A 94, 030302 (2016), Phys. Rev. A 91, 032326 (2015)]. In previous literatures, it has been shown that changing the device parameters needs an eavesdropping optical power of >0.25 W. For instance, a Si single photon detector under >0.25 W CW light observed temporal parameter change like efficiency, dark count rate, breakdown voltage, etc. [Phys. Rev. Lett. 112, 070503 (2014)]. The InGaAs PIN detector for monitoring shows changes in photosensitivity with >0.5 W CW light input [Phys. Rev. A 94, 030302 (2016)]. Moreover, attenuators, circulators and isolators also show parameter change with >1 W CW light input [Phys. Rev. Applied 13, 034017 (2020), arXiv:2201.06114]. Advantageously, the optical power limiter according to example embodiments regulates the energy of the output light, no matter how strong the input light is. As such, the optical power limiter according to example embodiments can provide an excellent protection for the calibrated components and devices. Advantageously, the small insertion loss introduces minimal degradation to the signal-to-noise ratio for the system, making the optical power limiter according to example embodiments suitable as a general component for both the transmitter and receiver protection.

In optical communications, regulating the optical power is also very important. The optical power limiter according to example embodiments can be useful in power equalization in wavelength division multiplexing (WDM) systems, erbium-doped fiber amplifiers (EDFA) gain control, receiver protection, etc.

In a WDM system, multiple wavelength channels arriving at a node may be transmitted through different optical passes and have different output power. Before the combined signals enter the optical amplifier, it is required that the optical power of these channels is equalized to maintain appropriate optical amplifier performance. This is typically done by actively monitoring and controlling the optical power [“MEMS variable optical attenuator (VOA) for DWDM applications.” Design, Test, Integration, and Packaging of MEMS/MOEMS 2002. Vol. 4755. SPIE, 2002. “Micromachined electromagnetic variable optical attenuator for optical power equalization.” Journal of Micro/Nanolithography, MEMS, and MOEMS 4.4 (2005): 041304.]. The optical power limiter according to example embodiments can provide automatic power control, with minimum insertion loss to the input signal. The optical power limiter according to example embodiments has great potential to complement or even replace the techniques in the power equalization in WDM systems.

6 FIG. 600 shows a schematic drawing of a power limiter arrayfor multi-channel applications according to an example embodiment.

Another issue in WDM networks is the wavelength-dependent gain saturation of the EDFA. When the input optical power becomes a significant fraction of the pump power, it will cause pump depletion and the reduction in amplifier gain. Because of this effect, the loss or removal of one or more channels at the input of an EDFA can cause large changes in the output power of the remaining channels [“Comparison of gain control techniques to stabilize EDFAs for WDM networks.” Optical Fiber Communications, OFC . . . IEEE, 1996., WIPO (PCT) WO2003014775A2]. With the optical power limiter according to example embodiments, one can limit the input power of all channels below the threshold, avoiding the EDFA gain saturation.

Also, the optical power limiter according to example embodiments is useful for receiver protection. As described earlier, if the channel happens to have high power optical input, the receiver performance could be altered, or the receiver could even be damaged. The optical power limiter according to example embodiments provides an automatic power regulation preventing such damage, with minimum losses on the optical signal in normal operating conditions.

300 3 FIG. The feasibility of using the optical power limiter according to example embodiments for optical device protection was experimentally verified. The experimental scheme is based on a pair of single-mode optical fibers and GRIN lenses in the optical power limitershown in. Experiments were conducted with four experimental configurations, as shown in the below table I, to test whether the optical power limiter according to an example embodiment can protect a photodetector from being attacked by externally injected strong lasers.

TABLE I Pulsed CW laser CW laser laser pulsed laser input input input input With power limiter Yes No Yes No in place Photodiode get No Yes No Yes damaged

Specifically, the laser attack experiment was conducted based on continuous-wave (CW) laser input and pulsed laser input. For the CW laser input, the optical power of the laser was gradually increased, sent through the optical power limiter according to an example embodiment, and injected into a fiber-coupled InGaAs photodiode. The attack was performed with different input optical power up to 1 W, and the responsivity (or quantum efficiency) of the photodiode under test remains unchanged.

The experiments were also conducted with the optical power limiter removed from the setup. By increasing the input CW optical power, and illuminate the light on the photodiode for 5s, it was observed that the responsivity of the photodiode starts decreasing when the input optical power goes above 300 mW. These changes to the photodiode are permanent and will modify the calibrated parameter of the system.

7 9 FIGS.to Also, experiments were conducted based on a series of pulsed laser inputs with strong peak power, to verify the effectiveness of the optical power limiter according to an example embodiment in these cases. To this end, two stages of EDFA were used to amplify the pulsed laser signal generated from a 1550 nm semiconductor laser, and the pulse widths (50-500ps) and the repetition rate (1 MHz~100 MHz) of the laser pulse were varied to generate various laser pulse configurations, with peak power ranging from 2.9 W to 14.7 W. In those experiments, no significant changes in photodiode responsivity were observed. The detailed responsivity measurement results with different input peak power in this experiment are shown in, illustrating substantially stable responsivity.

2 s 10 FIG. For pulsed laser input, the experiment was also conducted with the optical power limiter removed from the setup. By increasing the accumulative illumination time, and the input pulse fixed at 14.7 W peak power with a 1 MHz repetition rate, it was observed that the responsivity of the photodiode starts decreasing sharply afterillumination. The details of the experimental results are shown in.

11 FIG. The optical power limiter according to example embodiments can be a powerful solution for laser damage attack in quantum cryptography, and one can also expect more useful tools and methods can be built based on it. For example, since the calibrated parameter specs of a photodiode is reliable and could not be modified by an eavesdropper when using an optical power limiter according to an example embodiment, one can faithfully monitor the input optical power and perform feedback control to achieve 1) ultra-low power limiting threshold, 2) instantaneous optical power limiting. The schematic of the setup according to an example embodiment is shown in.

1100 1102 1104 1106 1108 1110 1106 1104 1102 1108 Specifically, the input optical signalwill go through a power limiter (PL)first, and is then split by a beam splitter (BS). Part of the energy will be sent to a monitoring photodiode (Mon PD), the rest will be delayed and attenuated (Electronic Variable Optical Attenuators, EVOA)before being output to a single photon detector (SPD) or Homodyne detector. Since the optical components (monitoring photodiode, beam splitter) are protected by the optical power limiterand their calibrated parameters are hence reliable, one can monitor the (averaged or instantaneous) input power of the optical signal, and actively control the EVOAto achieve a desirable output power.

Embodiments of the present invention can have one or more of the following features and associated benefits/advantages:

Feature Benefit/Advantage Miniature size The optical power limiter according to an example embodiment is able to achieve optical power limiting with a beam size of around micro-meter, enabling a small footprint and a high integration level for applications. Minimal insertion The optical power limiter according to an loss example embodiment optimizes the optical mode coupling in the low input power case. So the insertion loss in low input power scenarios are minimal. Adjustable power The optical power limiter according to an limiting threshold example embodiment provides configurable system parameters to adjust the power limiting threshold. Minimal distortion The power limiter according to an example on the signal embodiment adds only attenuation on the input optical signal, and introduce minimal impact (if not, negligible impact) on the intensity, phase, or polarization degrees of freedom of the input optical signal. Cost-effectiveness Composing of only off-the-shelf optical components and having a simple structure, the optical power limiter according to an example embodiment possesses the advantage of compact size, simple assembling, and cost-effective, etc.

The optical power limiter according to example embodiments can be used, for example, in the following applications: protecting optical components in optical communication and sensing systems, limiting eavesdropper's information in quantum cryptography applications, etc.

In one embodiment, an optical power limiter is provided comprising a first optical mode altering element configured to receive an input optical signal from a first waveguide; and an active medium coupled to the first optical mode altering element at a first end of the active medium such that a mode altered optical signal based on the input optical signal can enter the active medium, wherein a second end of the active medium is configured to couple the mode altered optical signal into a second waveguide as an optical output signal; wherein the active medium has a thermo-optic coefficient such that the mode altered optical signal entering the active medium experiences a refractive index gradient in the active medium as a result of absorption; and wherein the power of the optical output signal coupled into the second waveguide is limited to a maximum power value based on mode overlap of the mode altered optical signal and the second waveguide.

The maximum power value may be dependent on an optical path length between the first and second ends of the active medium.

The active medium may have a negative thermo-optic coefficient for diverging the light beam as a result of the refractive index gradient. As a result of the refractive index gradient, the active medium may introduce increased mode mismatch between the mode altered optical signal and the second waveguide with increased divergence of the mode altered optical signal.

The first optical mode altering element may comprise a core expansion in an optical fiber as the first waveguide, a taper structure, or large core waveguide mode converter in a photonic integrated circuit as the waveguide. The optical power limiter may comprise a second optical mode altering element coupled between the second end of the active medium and the second waveguide for optimizing mode overlap between the mode altered optical signal and the second waveguide up to the maximum power value.

The first optical mode altering element may be configured for focusing the mode altered optical signal in the active medium. The first optical mode altering element may comprise one or more of a group consisting of micro lenses, Gradient-Index (GRIN) lenses, collimators, and the waveguide versions of micro lenses, Gradient-Index (GRIN) lenses, collimators.

The optical power limiter may comprise a second optical mode altering element coupled between the second end of the active medium and the second waveguide for optimizing mode overlap between the mode altered optical signal and the second waveguide up to the maximum power value. The second optical mode altering element may comprise one or more of a group consisting of micro lenses, Gradient-Index (GRIN) lenses, collimators, and the waveguide versions of micro lenses, Gradient-Index (GRIN) lenses, collimators.

12 FIG. 1200 1202 1204 1206 shows a flowchartillustrating a method of fabricating an optical power limiter, according to an example embodiment. At step, a first optical mode altering element is configured to receive an input optical signal from a first waveguide. At step, an active medium is coupled to the first optical mode altering element at a first end of the active medium such that a mode altered optical signal based on the input optical signal can enter the active medium. At step, a second end of the active medium is configured to couple the mode altered optical signal into a second waveguide as an optical output signal, wherein the active medium has a thermo-optic coefficient such that the mode altered optical signal entering the active medium experiences a refractive index gradient in the active medium as a result of absorption; and wherein a power of the optical output signal coupled into the second waveguide is limited to a maximum power value based on mode overlap of the mode altered optical signal and the second waveguide.

The maximum power value may be dependent on an optical path length between the first and second ends of the active medium.

The active medium may have a negative thermo-optic coefficient for diverging the light beam as a result of the refractive index gradient. As a result of the refractive index gradient, the active medium may introduce increased mode mismatch between the mode altered optical signal and the second waveguide with increased divergence of the mode altered optical signal.

The first optical mode altering element may comprise a core expansion in an optical fiber as the first waveguide, a taper structure, or large core waveguide mode converter in a photonic integrated circuit as the waveguide. The method may comprise coupling a second optical mode altering element between the second end of the active medium and the second waveguide for optimizing mode overlap between the mode altered optical signal and the second waveguide up to the maximum power value.

The method may comprise configuring the first optical mode altering element for focusing the mode altered optical signal in the active medium. The first optical mode altering element may comprise one or more of a group consisting of micro lenses, Gradient-Index (GRIN) lenses, collimators, and the waveguide versions of micro lenses, Gradient-Index (GRIN) lenses, collimators. The method may comprise coupling a second optical mode altering element between the second end of the active medium and the second waveguide for optimizing mode overlap between the mode altered optical signal and the second waveguide up to the maximum power value. The second optical mode altering element may comprise one or more of a group consisting of micro lenses, Gradient-Index (GRIN) lenses, collimators, and the waveguide versions of micro lenses, Gradient-Index (GRIN) lenses, collimators.

In one embodiment, an optical device or system comprising the power limiter of the above embodiments is provided.

In one embodiment, a method of limiting optical power using the power limiter of the above embodiments is provided.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive. Also, the invention includes any combination of features described for different embodiments, including in the summary section, even if the feature or combination of features is not explicitly specified in the claims or the detailed description of the present embodiments.

In general, in the following claims, the terms used should not be construed to limit the systems and methods to the specific embodiments disclosed in the specification and the claims, but should be construed to include all processing systems that operate under the claims. Accordingly, the systems and methods are not limited by the disclosure, but instead the scope of the systems and methods is to be determined entirely by the claims.

Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in a sense of “including, but not limited to.” Words using the singular or plural number also include the plural or singular number respectively. Additionally, the words “herein,” “hereunder,” “above,” “below,” and words of similar import refer to this application as a whole and not to any particular portions of this application. When the word “or” is used in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list and any combination of the items in the list.